Research this past year was focussed on the molecular characterization of excitatory amino acid receptors and their function in the auditory system. Using in situ hybridization histochemistry, the expression of glutamate receptor subunits in morphologically-identified neurons in the cochlear nucleus was determined. These studies suggest that glutamate receptors are functional at synapses between auditory nerve and large neurons in the cochlear nucleus, synapses between parallel fibers and small neurons in the DCN, and synapses on granule cells made by an input yet to be determined. Based on these findings we conclude that these are synapses where an excitatory amino acid is used as a neurotransmitter. These three synaptic populations contain different compositions of glutamate receptor subunits, and the subunit composition may vary within the auditory nerve synapse population. Using anti-peptide antibodies, we have shown that the glutamate receptor is a hetero-oligomer made up exclusively of GluR1-4 subunits in rat brain. An Mr=590k was the largest complex observed and is consistent with its being a pentamer of the Mr = 108k monomer. Immunocytochemical analyses showed different but overlapping distributions of the receptor subunits. Electron microscopy of immunolabeled sections showed labeling of postsynaptic densities and cytoplasm. No evidence of presynaptic labeling was seen. These studies are the first to characterize the biochemical properties of the glutamate receptor subunits and conclusively show a postsynaptic localization of this receptor. Analysis of chimeras of the glutamate receptor and kainate binding protein shows that amino acids 70-170 are important for kainic acid binding. Analysis of chimeras for ion channel activity using the oocyte expression system showed that one chimera had weak activity activated by glutamate. Analysis of the kainate binding protein gene from chick brain showed it to be composed of nine exons extending over 13 kb of genomic DNA. Although the predicted protein has a membrane topology similar to that of ligand- gated ion channel subunits, the exon-intron organization of the kainate binding protein was different. Two distinct forms of cDNA encoding new members of a dipeptidyl aminopeptidase were isolated. In rats, one of the cDNAs encodes a protein (DPPX-S) of 803 amino acid residues while the other encodes a protein of 859 (DPPX-L) residues. DPX-S mRNA is expressed in brain and some peripheral tissues while distribution of DPPX-L in brain suggests that it may be related to regionally localized events, such as metabolism of a neurotransmitter. This protein has several properties of a protease which degrades N-acetylaspartyl glutamate, a putative excitatory amino acid neurotransmitter.